Breathing normobaric carbogen (95% O2/5% CO2) has been shown to increase oxygenation in some tumors, making them more radiation sensitive, and has resulted in improved drug delivery. We have examined two experimental murine tumors for their oxygenation response to carbogen breathing: the radiation-induced fibrosarcoma RIF-1 and the mammary adenocarcinoma MTG-B. Using MR imaging techniques sensitive to hemoglobin oxygenation (Blood Oxygen Level Dependent commonly named BOLD), one can determine non invasively if there is a vascular response to carbogen breathing. A multi-echo FLASH sequence was used on a 7 Tesla magnet to quantify the apparent transverse relaxation time T2* on a pixel by pixel basis. T2* map variation will then reflect deoxyhemoglobin content changes. RIF-1 were also assessed for pO2 using EPR oximetry. A 1.2GHz spectrometer and LiPc crystal as the oxygen sensitive material were used. The EPR oximetry measurements showed little change in pO2 with carbogen breathing in these two tumors. The T2* response with carbogen breathing showed significant variability between and within tumors when the whole tumor was used as a region of interest. In RIF-1 tumors the T2* tended to decrease, an effect which is opposite to that expected if carbogen increased blood oxygenation in the tumor. These tumors appear to exhibit a vascular response which results in increased deoxyhemoglobin with carbogen (due to increased blood volume and/or decreased hemoglobin saturation). Using T2* mapping we can quantify the response of a tumor to carbogen with high spatial resolution. There is a significant inter-tumor variability, and T2* mapping may be able to identify tumors that can be reoxygenated with carbogen. The macroscopic variation provides regional information on tumor oxygenation and vascular regulation.